Definition

Bifascicular block is defined as “a conduction defect of two of the three main fascicles of the His/Purkinje system located below the atrioventricular (AV) node”⁴.

It is widely accepted that bifascicular block is classified as a right bundle branch block (RBBB) combined with either a left anterior fascicular block (LAFB)or left posterior fascicular block (LPFB). However, some authors also consider bifascicular block to include a complete LBBB (i.e. affecting both the LAFB and LPFB).

Historically, the term trifascicular block has been used when bifascicular block is associated with 1^st, 2^nd or 3^rd degree AV block. However, as 1^st and 2^nd degree blocks are due to AV node pathology rather than a fascicular conduction defect, this is a misnomer. A true trifascicular block requires a bifascicular pattern on ECG plus a complete (3^rd degree) heart block, reflecting the fact that all three fascicles are disrupted.

N.B. Some authors refer to bifascicular block with 1^st or 2^nd degree block as “incomplete trifascicular block” and bifascicular block with 3^rd degree block as “complete trifascicular block”⁵. 

Learning bite

Bifascicular block is a conduction defect of two of three main fascicles of the His/Purkinje system located below the AV node.